The invention relates to vertical kilns or dryers, used for drying products such as panels or articles of timber or other material, for example after painting, coloring, impregnation or other types of operation. Equipment of this type contains, within a parallelepipedal heating chamber, two or more adjacent stacks of horizontal trays, which are carried by an elevating and lowering system and by a translating device along a zig-zag path which usually starts at the base of the first stack with an ascending movement, at the point where the trays are loaded with the articles to be dried, and which terminates at the base of the last stack with a descending movement, at the point where the dried articles are unloaded from the trays. A final translating device then transfer the empty trays directly from the lower part of the last stack to the lower part of the first stack, thus forming the path of the trays into a closed loop.
To gain a clearer idea of the problems encountered in vertical kilns of the known type, reference is made to FIGS. 1 and 2 which show, schematically and in a theoretical way, a known kiln with two stacks of trays, seen from the side and from the front of one of the stacks respectively. The trays V of the stacks C1 and C2 are, for example, of rectangular shape and have their short sides resting on brackets M carried by opposite vertical pairs of chain conveyors T1 and T2, which are synchronized with each other and provided with an intermittent and opposing motion, to elevate the trays in the stack C1 and lower those in the stack C2. The brackets of the conveyors T1 and T2 are aligned horizontally with each other, and thus when a tray reaches the top of the stack C1 it can be transferred to the top of the stack C2 by means of horizontal chain conveyors T3, parallel to the path of translation, which have teeth D which push the long side of the tray and transfer it from the brackets of T1 to the coplanar brackets of T2.
The tray which cyclically reaches the bottom of the stack C2 first has the dried articles unloaded from it at a station K2 and is then placed on a horizontal chain conveyor T4 which transfers the empty tray at the correct time from C2 to C1, where means operate at the station K1 to load new articles for drying on to the empty tray. The stages of transfer of the trays between the two stacks take place at the same time as the stages of loading and unloading other trays at the stations K1 and K2.
In a known kiln of this type or of a similar type, the following drawbacks are encountered. The presence of the chains of the conveyors T1 and T2 throughout the height of the stacks C1 and C2, and especially the presence of the conveyors T3 located transversely and above the trays of C1 and C2, can cause dirt to fall on the articles contained in the trays, thus inevitably leading to the rejection of products.
The whole of the weight of the trays contained in the stacks C1 and C2 is discharged on to the upper return shafts of the conveyors T1 and T2, and these shafts must therefore be supported by a frame which has suitable load-bearing characteristics, and is therefore relatively heavy, bulky and expensive.
As shown in FIG. 2, the conveyors T1 and T2 have a considerable depth, and make it necessary to provide means of circulating the hot drying air at a considerable distance from the ends of the trays located in the stacks C1 and C2, with markedly adverse effects on the functionality of these means.
The chains which form the elevating and lowering conveyors T1, T2 are mechanisms which, although very reliable from the technological point of view, are expensive and require periodic maintenance.
When the known conveyors T1 and T2 for elevating and lowering the trays are used, it is very difficult to apply the method of short-cycle movement of the trays, described in Italian patent application No. BO 99A 000089 in the name of the applicant, to which reference will generally be made, according to which the trays are elevated by at least one step from the loading station and then the trays are translated and lowered directly to the unloading station from which the trays are returned towards the loading station with the usual lower translating means. All the other trays not included in the short cycle must remain stationary.
The invention is designed to overcome these and other drawbacks of the vertical kilns according to the known art, by the following idea for a solution.
The trays are provided at their corners, for example at the ends of the long sides, with vertical spacers, projecting upwards for example, which are identical and enable stacks to be formed in which the trays rest on one another and are spaced apart with a desired interval. The spacers are preferably such that their upper ends interact with suitable seats formed on the bases of the trays lying above, in such a way that the stacked trays are perfectly centered with respect to each other. In this case, the rising and descending movement of the trays in the consecutive stacks of the kiln is achieved by means of elevating and lowering devices which act on the bottom tray of each stack, grippers being provided in the stacks with ascending movement to retain the bottom tray of these stacks in position, to enable the ascending means to return to the low position for the repetition of a new cycle. In the stacks with a descending movement, similar grippers are provided to keep the tray next to the bottom held in the high position, while the elevating and lowering device supports the bottom trays of the stacks on the lower translating device such as a conveyor. The trays, held by the gripper, are at such a height that they do not interfere with the lower tray translated by the lower translating device. For the upper translation of the trays from the top of one stack to that of the adjacent stack, it is possible to provide chain conveyors parallel to the short sides of the trays, outside the stacks of the trays, and it is possible to associate these conveyors with means which, at the appropriate time, slightly raise the trays to be translated and which then translate the trays and deposit them on the trays of the receiving stack.
Clearly, the new solution makes it possible
to eliminate the conventional elevating and lowering chain conveyors T1 and T2 (FIGS. 1 and 2) and to overcome all the drawbacks arising from the use of such conveyors;
to place the conveyors for translating the trays from one stack to the adjacent stack outside the plan dimensions of the stack of trays and thus to eliminate the drawbacks arising from the use of the conventional translating devices of the type shown by T3 in FIGS. 1 and 2;
if the kiln is to be used for operation with a short cycle, it is simple to place a supplementary translating device, similar to the upper means, at the desired height of the stacks of trays, to move a limited number of trays along a short path closed in a loop between the loading and unloading stations, and it is simple to use the supplementary elevating device to raise the stacks of trays not included in the short path, to release them from the action of the lower elevating and lowering device. The auxiliary translating device for the short cycle is also located outside the plan dimensions of the trays and cannot deposit dirt on the dried articles.